1. Field of the Invention
The present invention relates to a method for preparing an analytical standard used for an elemental analysis utilizing microbeam, and an analytical standard prepared by the method.
2. Description of the Related Art
Conventionally, as a technique for a local elemental analysis, there can be mentioned a microbeam X-ray fluorescence analysis in which: a sample is irradiated with sharp excitation light of from a several hundred-nanometer to several-micrometer radius; a type of element is specified based on energy of the generated X-ray fluorescence; and abundance of the element is determined based on the intensity of the generated X-ray fluorescence. The microbeam X-ray fluorescence analyses are classified into several categories depending on the type of the excitation light. Among these, because of less damage on the sample during measurement, attentions have been paid to a synchrotron radiation X-ray fluorescence analysis (SR-XRF) and a proton (particle) induced X-ray emission analysis (PIXE), and they have been widely used. The microbeam X-ray fluorescence analysis has been utilized in various fields, including: a homogeneity test of metal evaporated onto electronic parts (material engineering); an analysis of element localization in mineral samples (earth science); detection of pollutant element in environmental samples, such as suspended particles in the atmosphere (environmental science); and examination of nutritional state by hair analysis or distribution of trace element in tissues (medical science) and the like. In recent years, the detection sensitivity has remarkably improved by the introduction of large facilities for microbeam analysis, and accordingly the utilization of microbeam X-ray fluorescence analysis in the biomedical field has been remarkably boosted.
In the microbeam X-ray fluorescence analysis, an analytical standard (which may be also called “analytical standard sample” or “analytical standard preparation”) is used. Herein, the analytical standard means a sample to be used as a standard in a series of measurements for the purpose of measurement accuracy control and quantification. The analytical standard contains an element of a known concentration, and the concentration of the sample of interest can be obtained, for example, from the intensity of the X-ray fluorescence of the sample of interest, by comparing with the intensity of the X-ray fluorescence of the analytical standard at different concentrations as reference. Therefore, it is desirable that the physical properties of the analytical standard be similar to those of the sample to be measured.
For the analytical standard, for example, there has been known a sliced glass containing an element of a known concentration, as well as a standard prepared by evaporating metal onto a thin film (hereinafter, referred to as “analytical standard of evaporation type”), and a disclosure has been made that these analytical standards are applied to the microbeam X-ray fluorescence analysis (see, for example, Non-Patent Document 1). Herein, the expression “evaporating metal” means a treatment in which a metal is vaporized and attached to a surface of a substrate.
Further, for the analytical standard, for example, there has been known an analytical standard obtained by dropping metal solution on a filter paper and drying the filter paper (hereinafter, referred to as “analytical standard by droplet drying method”), and a disclosure has been made that the analytical standard is applied to the microbeam X-ray fluorescence analysis (see, for example, Non-Patent Document 2).
Non-Patent Document 1: M. Watanabe, and D. B. Williams, Atomic-level detection by X-ray microanalysis in the analytical electron microscope, Ultramicroscopy 78 (1999) 89-101
Non-Patent Document 2: K. Watanabe, O. Miyakawa, and M. Kobayashi, New method for quantitative mapping of metallic elements in tissue sections by electron probe microanalyzer with wavelength dispersive spectrometers, Journal of Electron Microscopy 50 (2001) 77-82
The analytical standard of evaporation type disclosed in the Non-Patent Document 1 can be applied to fields, such as material engineering and measurement engineering. However, it is often the case that such an analytical standard cannot be applied to the measurement of biological samples of interest in biomedical field. In other words, the amount of the element present in a biological sample to be measured frequently is as small as several ppm or less, which leads to a large gap in the element content between the biological sample to be measured and the analytical standard of evaporation type. In addition, the biological sample to be measured usually has high transparency to excitation light, which also leads to a notable difference in the transparency to excitation light between the biological sample to be measured and the analytical standard of evaporation type.
Moreover, the analytical standard by droplet drying method disclosed in the above-mentioned Non-Patent Document 2 may not be used as an analytical standard, depending on the types of the element. For example, when mercury is dropped, mercury itself may be vaporized depending on its chemical form. This may result in, in a drying step after dropping, an uneven element distribution at the portion where mercury was dropped. As a result, it becomes difficult to obtain an analytical standard containing mercury of a desired concentration.
Therefore, the object of the present invention is to solve the above-mentioned problems and to provide an analytical standard used for an elemental analysis utilizing microbeam and a method for preparing the analytical standard.